Studies on the Water Relations of Pinus sylvestris in Plantation Conditions. IV. Direct Observations on the Rates of Transpiration, Evaporation of Intercepted Water, and Evaporation from the Soil Surface

Abstract

The total evaporation from a plantation of P. sylvestris was estimated on 44 randomly chosen sample days, from measurements of the water lost from isolated blocks of soil within the plantation, and of the loss of weight of detached branches or leaves, suspended in theplaces from which they had been removed, during the 10 min. after they were detached. The effect of detachment on the rate of water loss from branches and leaves was probably negligible in the conditions of these experiments. The branches or leaves were sampled so as to represent different layers of the canopy and different aspects. The rates of water loss from themwere expressed per unit weight of leaf, and the amounts of leaf present per hectare in the plantation were determined by a sampling procedure, from which the pattern of change in the weight and area of the canopy couldbe determined. The dry weight of the foliage varies from 5000-6000 kg/ha at the end of winter to 9000-10,000 kg/ha in summer, and the leaf area index (total surface) appears to vary seasonally between about 2.5 and 4.5. The total evaporation from the plantation exceeded Eo (potential evaporation from an open water surface) on rainy days (when the foliage was wet). It was less than Eo on rainless days from Nov. to May but it was not significantly different from E during June to Oct. The mean evaporation determined in this way was similar in amount to the mean evaporation estimated from the rainfall during the periods of soil water deficit in the 3 years.